The Encroachment is about to dim the sun of Serehfa Fastness, where the characters live on and in a mysterious piece of mega-architecture, and plunge the world into an ice age. The King and his advisers know this, but they continue to fight the clan of Engineers. The crypt also knows of the threat, and sends an emissary.
The book is set on a far future Earth where the transfer of mindstates into a world-spanning computer network (known as "the data corpus", "cryptosphere" or simply crypt) is commonplace, allowing the dead to be easily reincarnated (though by custom, only a limited number of reincarnations are allowed).
Humanity has lost much of its technological background, due partly to an exodus by much of the species, and partly to the fact that those who remained (or at least their rulers) are fighting against more advanced technology such as Artificial Intelligence.
Meanwhile, the solar system is drifting into an interstellar molecular cloud ("the Encroachment"), which will eventually dim the Sun's light sufficiently to end life on Earth. However, the Ancients have left behind a device (the "Fearsome Engine" of the title) to deal with the problem; the book follows four characters who become involved in the attempt to activate it, with the narrative moving between the four (who do not meet until very near the end) in rotation.
'Woak up. Got dresd. Had brekfast. Spoke wif Ergates thi ant who sed itz juss been wurk wurk wurk 4 u lately master Bascule, Y dont u ½ a holiday? & I agreed & that woz how we decided we otter go 2 c Mr Zoliparia in thi I-ball ov thi gargoyle Rosbrith.'